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Old 01-25-2014, 02:42 PM   #1
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How much weight (at the middle) will an 11 ' douglas fir 4X4 carry? Supported at the ends.


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Old 01-25-2014, 11:44 PM   #2
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Maybe one child on a swing.



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Old 01-26-2014, 09:32 AM   #3
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Depends how much deflection you are prepared to tolerate.
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Old 01-26-2014, 10:12 AM   #4
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According the the Forestry Forum (http://www.forestryforum.com/members/donp/Fblist.htm), douglas fir has an allowable maximum fiber bending stress ranging from 875 psi for #2 to 1850 psi for dense select structural. Let's take the worst and the best case for you. Assume that your 4x4 is actually 3.5 inches by 3.5 inches.

Worst Case 875psi: 190 lbs at the midpoint
Best case 1850 psi: 400 lbs

These values provide a factor of safety of 1.0 in bending. This does NOT analyze for deflection, which may be important in your case. This does NOT analyze for shear, which may be important. This analyzes for a STATIC load only, does NOT account for dynamic loads such as a swinging load. This does NOT account for repetitive loading or fatigue, which may be important in your case.
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Old 01-28-2014, 08:53 AM   #5
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load bearing

Those are some good numbers Daniel.......good to know.

What's important to GLK is the weight this will support at mid span..........I'd stick to the lower half...........200 - 300 lbs.
Provided there's not a big knot anywhere near the mid span, which could weaken the 4x4 a lot.
If this is a dynamic load ( a swing for instance) what's your best guess Daniel for reducing that weight rating ?

Also I think deflection is more of a concern than shear.......providing there's proper bearing support at the ends.......at least 2" .......3" would be better.

Last edited by Gary Evans; 01-28-2014 at 08:57 AM.
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